JPH11163833A - Optical transmission method - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To prevent the deterioration of transmission quality by defining the wavelength interval of each light source for a fixed intensity modulation level. SOLUTION: The optical signals from each of the optical transmission devices 1 to n are wavelength-multiplexed and transmitted to an optical fiber 30, received by a light receiver 21 of a center station 20 and distributed to a plurality of light by a distributor 22. In the optical transmission method of demodulating by the demodulator 23, each optical transmission device sets the wavelength interval of the light source to the center frequency of the optical beat noise and the modulator 13 when the effective modulation degree is 40% or less, for example.
The difference from the frequency of the electric signal from
An optical signal is output from each light source 17 by setting the distance apart by Hz or more to reduce optical beat noise.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical CATV and optical I
The present invention relates to an optical transmission method used for a TV or the like, and more particularly, to an optical transmission method for setting a wavelength interval of a light source and transmitting the light.

[0002]

[Related Background Art] Conventionally, this type of optical transmission method includes:
For example, as shown in Japanese Patent Application No. 6-41902, optical signals that are intensity-modulated from a plurality of light sources and output are combined by an optical coupler, transmitted to a single optical fiber, and the optical signals are collectively collected. In some cases, light is received by a photodetector composed of one photodiode (PD) and demodulated by a demodulator.

In this method, a plurality of optical signals are converted into one PD.
As a result, an interference component called optical beat noise occurs around a frequency corresponding to the wavelength difference of the optical signal. When the light from the light source is intensity-modulated by the modulated signal, the spectrum of the optical beat noise fluctuates in the frequency direction and spreads (wavelength chirping). The transmission quality of the signal transmitted in this way is evaluated based on noise level versus signal level characteristics.

For this reason, if the light source emits light (hereinafter simply referred to as “wavelength”) into the system without selecting it, the wavelength difference between the wavelengths of adjacent light sources is narrow, and the center frequency of the optical beat noise is reduced. Is a relatively low frequency band,
For example, the noise level may occur in a transmission band, and the influence thereof may greatly deteriorate the noise level versus signal level characteristic. Therefore, in the above-mentioned conventional method, in order to avoid this effect, the wavelengths of the respective light sources are selected and the wavelengths between the light sources are arranged so that the deterioration of the relative intensity noise (RIN) is minimized.

[0005]

However, in the conventional system, only the influence of the optical beat noise in the case where one modulated signal is intensity-modulated for each light source is considered. In the case where an intensity-modulated optical signal is output from the light source, the optical signal is transmitted to an optical fiber via a new optical coupler, which may increase transmission loss and manufacturing cost. is there.

Therefore, it is conceivable to use a plurality of light sources to modulate the intensity of the light from the light sources using a frequency-multiplexed modulated signal. This causes a problem that the transmission quality of the transmitted signal is degraded due to the occurrence of the error. The present invention has been made in view of the above problems, and has as its object to provide an optical transmission method capable of defining a wavelength interval of each light source for a constant intensity modulation level and preventing deterioration of transmission quality. And

[0007]

In order to achieve the above object, according to the present invention, light from each transmission unit is modulated by a plurality of different frequency-multiplexed electric signals, and output as an optical signal. In the optical transmission method in which the optical signal is coupled and transmitted to the optical transmission line, and the light is received by the light receiving unit and demodulated, each transmission unit sets the wavelength interval of the output light based on the effective modulation degree, and converts the optical signal. An output optical transmission method is provided.

That is, in each transmission section, when a constant intensity modulation level (effective modulation degree), for example, the effective modulation degree is 40% or less, the wavelength interval of the light source is adjusted so as not to degrade the transmission quality. Is preferably set so that the difference between the center frequency of the electric signal and the frequency of the electric signal closest to the optical beat center frequency among the electric signals is separated by 20 GHz or more, thereby reducing optical beat noise.

Further, for example, when the effective modulation degree is 30% or less, the wavelength interval of the light is the difference between the center frequency of the optical beat noise and the frequency of the electric signal closest to the optical beat center frequency among the electric signals. Are preferably set so as to be separated by 15 GHz or more. Further, in the transmission unit, of the electric signals of different frequencies for modulating light, the wavelength interval of light is set based on the difference between the frequency closest to the center frequency of optical beat noise and the center frequency of optical beat noise. Is preferred.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical transmission method according to the present invention will be described with reference to FIGS. FIG. 1 is a configuration diagram showing a first embodiment in which the optical transmission method according to the present invention is used in an optical ITV system. In the figure, in the above system, a plurality of optical transmission devices 1 to n constituting a transmission unit according to the present invention are shown.
And a center station 20 that constitutes a light receiving unit according to the present invention. Each of the optical transmission devices 1 to n and the center station 20 are connected via an optical fiber 30. Here, n is any positive number.

The optical transmission apparatus 1 constitutes a predetermined transmission section according to the present invention, and generates electric signals having different frequencies having information such as video and audio from a plurality of signal generators 10 to 12. Signals from the signal generators 10 to 12 are modulated by modulators 13 to 15, respectively. The plurality of modulated signals from the modulators 13 to 15 are multiplexed by the mixer 16, and the light source 17 including, for example, a laser diode is
The multiplexed signal is subjected to intensity modulation to output an optical signal. This optical signal is output to the optical fiber 30 via the optical coupler 18.

Each of the optical transmission devices 2 to n includes one signal generator 10 and a modulator 19 in the configuration of the optical transmission device 1.
, A light source 17 and an optical coupler 18.
Optical signals from the optical transmission devices 1 to n are multiplexed by the optical couplers 18 and transmitted to the center station 20 via the optical fiber 30. In the center station 20, a photodetector 21 composed of, for example, a photodiode collectively receives the multiplexed optical signals from the connected optical fiber 30 and performs optical / electrical conversion. The electric signal is distributed to a plurality of signals by the distributor 22, and the demodulator 23 demodulates the distributed electric signal for each frequency assigned to the electric signal.

The light source 17 of the optical transmission device 1, which is intensity-modulated by the frequency-multiplexed modulated signal, is generally intensity-modulated at an effective modulation degree of 40% or less so that clipping distortion does not occur. Is desirable. Therefore, in this embodiment,
For example, a case where the light source 17 is intensity-modulated at the effective modulation degrees of 40% and 30% will be described below. Here, the optical beat noise generated due to the interference between the light from the multiplexed modulated signal and the light from the intensity-modulated light source whose effective modulation degree is 40% and 30% and another light source is as shown in FIGS. Become. FIGS. 2 and 3 show an example in which the center frequency of the optical beat noise and the transmission carrier frequency of the electric signal from the signal generators 10 to 12 are substantially the same.

When this waveform is evaluated by RIN, it can be expressed as shown in FIG. In FIG. 4, the offset frequency is a frequency obtained by offsetting the frequency at which RIN is measured at the center frequency of the optical beat noise, that is, the center frequency of the optical beat noise and the frequency of the electric signal from the signal generator (the above center frequency (A frequency of a close electric signal).

Here, for example, the signal frequencies of the modulators 13 to 15 and 19 of each of the optical transmission devices 1 to n are set to F1 to FN, respectively.
And the wavelength of the light from each light source 17 is λ1
Assuming that λn, the center frequency fbeat of optical beat noise between adjacent wavelengths is: fbeat = c | λi−λi + 1 | / λiλi + 1 (1) where c is a constant and 3.0 × 10 ⁸ mi: 1 to n−1.

In this embodiment, RIN shown in FIG.
It is assumed that a value of 30 dB / Hz or less is secured. For this purpose, when the effective modulation degree is 40% or less, the multiplexing is performed so that the offset frequency has a relationship of | Fj−fbeat |> 20 GHz (2) By setting the wavelength interval between the light source of the optical transmission device 1 modulated with the modulated signal and the light source of another optical transmission device (optical transmission device 2), a signal level with good transmission quality can be obtained.

When the effective modulation degree is 30% or less, even if the above-mentioned wavelength interval is set so that the offset frequency has a relationship of | Fj-fbeat |> 15 GHz (3), the signal level is transmitted. Good in quality. Here, j is 1 to N
And N ≠ n is a positive number. Note that N ≠ n is equal to the number n of light sources because some light sources are modulated by a large number of modulators.
And the number N of modulators are not equal.

Therefore, when the effective modulation degree is 40% or less, for example, if the highest frequency signal is 1 GHz among the signal frequencies of F1 to FN, fbeat is 2 from the relation of the equation (2).
It can be seen that the wavelength interval should be set so as to be generated at 1 GHz or more. Thereby, for example, the light source is 1.31 μm.
When the m band is used, that is, when λi = 1310 nm, from equation (1), λi + 1 = 1310.13 nm and fb
eat = 22.7 GHz, which satisfies the relationship of equation (2). For example, when a 1.55 μm band is used as a light source, that is, when λi = 1550 nm,
From equation (1), fbeat = λi + 1 = 1550.17 nm
21.2 GHz, which satisfies the relationship of equation (2).

Further, when the effective modulation degree is 30% or less, for example, assuming that the highest frequency signal is 1 GHz among the signal frequencies of F1 to FN, fbeat is 2 from the relation of the equation (2).
It can be seen that the wavelength interval should be set so as to be generated at 1 GHz or more. Thereby, for example, the light source is 1.31 μm.
When the m band is used, that is, when λi = 1310 nm, from equation (1), fb is obtained at λi + 1 = 1310.10 nm.
eat = 17.5 GHz, which satisfies the relationship of equation (3). For example, when a 1.55 μm band is used as a light source, that is, when λi = 1550 nm,
From equation (1), fbeat = λi + 1 = 1550.13 nm
16.2 GHz, which satisfies the relationship of equation (3).

As described above, in this embodiment, as described above,
The influence of optical beat noise is avoided by setting the wavelength interval of the light source such that the value of RIN is an offset frequency that ensures a value of -130 dB / Hz or less. Therefore, in the present embodiment, even when there is a light source that is intensity-modulated by a large number of frequency-multiplexed modulated signals, the wavelength interval with the other light sources is set based on the effective modulation degree, so that the influence of the optical beat noise is obtained. Can be alleviated, and deterioration of transmission quality can be prevented.

In the present embodiment, a system including one light source whose intensity is modulated by a frequency-multiplexed modulated signal has been described. However, the present invention is not limited to this. Even when a plurality of offset frequencies exist, when the effective modulation degree is 40% or less, the offset frequency becomes 2
If the wavelength intervals of the respective light sources are set so as to be separated by 0 GHz or more and the effective modulation degree is 30% or less, and the offset frequency is separated by 15 GHz or more, the signal level sufficiently satisfies the transmission quality.

FIG. 5 shows an HFC according to the present invention.
FIG. 9 is a configuration diagram showing a second embodiment used in an optical CATV system of the (Hybrid Fiber Coaxial) transmission system. In the figure, a head end 40 on the center station side includes a downstream light source 41, an upstream receiver 42, and the like.
A downstream optical trunk line (optical fiber) 51 is connected to the upstream receiver 42, and an upstream optical trunk line (optical fiber) 52 is connected to the upstream receiver 42.

A plurality of optical couplers 61 are connected to the downstream optical trunk transmission line 51, respectively.
1 is branched by each optical coupler 61, and the other end of each of the branched transmission paths is connected to the downstream optical receivers 62 of a plurality of optical transmission apparatuses 1 to n on the subscriber side. A plurality of optical couplers 63 are respectively connected to the upstream optical trunk transmission line 52, and the upstream optical trunk transmission line 52 is branched by each optical coupler 63, and is connected to the other end of each of the branched transmission lines. Is connected to upstream light sources 64 of a plurality of optical transmission devices 1 to n on the subscriber side.

The downstream optical receiver 62 and the upstream light source 64 of each of the optical transmission devices 1 to n are connected to a diplex filter 65, and the diplex filter 65 is connected to each subscriber side via a coaxial cable network 66. Are connected to areas 71 to 7n in which terminal devices (not shown) are arranged. In the system of this embodiment, the upstream signal is transmitted to each area 71.
.. 7n by different frequencies. For example, in the area 71, the upstream signal is divided into the frequency band L1
In the areas 72 to 7n, the uplink signals use the frequency bands L2 to Ln, respectively.

The downstream signal from the downstream light source 41, which has been intensity-modulated by the communication from the head end 40 and the information signal, is transmitted via the downstream optical trunk line 51. The downstream signal is branched by the optical coupler 61, converted from the optical signal into an electric signal by the downstream light receiver 62, and is converted into a diplex filter 65.
, And transmitted to the respective areas 71 to 7n via the coaxial cable 66.

Up signals from the areas 71 to 7n are transmitted via the coaxial cable 66. The upstream signal is supplied to the upstream light source 6 by the diplex filter 65.
The light is separated into four light sources, and the upstream light source 64 is intensity-modulated. The intensity-modulated light from the upward light source 64 is supplied to each area 71
The wavelengths are set to different wavelengths, for example, λ1 to λn, respectively, and the respective lights are multiplexed by the optical couplers 63 and received by the upstream optical receiver 42 via the upstream optical trunk line 52. Is transmitted from each area 71 to 7n on the subscriber side to the head end.

Here, the reason why the intensity is modulated at a different frequency for each area is so that the signal of each area can be distinguished by each light receiver 42. In addition, it is desirable that the wavelengths of the respective light sources be different wavelengths as far as possible from each other in consideration of optical beat noise.
Therefore, the wavelength interval from the adjacent upstream light source 64 is RI
The offset frequency is set so that the value of N becomes an offset frequency that secures a value of -130 dB / Hz or less, that is, as in the first embodiment, when the effective modulation degree is 40% or less, the offset frequency becomes 20 GHz or more. When the effective modulation degree is 30% or less, the offset frequency becomes 15%.
The influence of optical beat noise is avoided by setting to be equal to or higher than GHz. Also in this case, the difference between the frequency closest to the center frequency of the optical beat noise and the center frequency in each frequency band is set as the offset frequency.

Therefore, in this embodiment, it is possible to alleviate the influence of the optical beat noise, prevent the deterioration of the transmission quality, and collectively transmit the signals to one optical trunk line using an optical coupler. Therefore, the total transmission distance of the transmission path can be reduced as compared with the conventional HFC transmission system. Further, in this embodiment, the number of downstream light sources and upstream light receivers on the head end side can be reduced, so that the number of head end components can be reduced and maintenance of each component can be facilitated.

FIG. 6 shows a third example in which the optical transmission method according to the present invention is applied to an optical CATV system of a double split transmission system.
It is a block diagram which shows an Example. This embodiment is different from the second embodiment in that the diplex filter 67 of each of the optical transmission devices 1 to n is configured so that the frequency band of the upstream signal can pass through two bands, and is separated. Each of the upstream signals is again multiplexed by the mixer 68 and output to the upstream light source 64. The other components are configured in the same manner as in the second embodiment, and thus are denoted by the same reference numerals.

For example, in a frequency band as shown in FIG.
The case where the frequency bands Le1 to Len and Lh1 to Lhn of the uplink signal are used and the uplink signals of two uplink frequency bands are used for each of the areas 71 to 7n will be described below. In FIG. 6, for example, the diplex filter 6 of the optical transmission device 1
7, the upstream signals pass through the frequency bands Le1 and Lh1, and the diplex filters 67 of the optical transmission devices 2 to n pass through the frequency bands Le2 to Len and Lh2 to Lhn. In this embodiment, the respective signals separated by the diplex filter 67 are multiplexed again by the mixer 68, and the multiplexed signal is used as the upstream light source 6
4 is intensity-modulated. The modulated light is applied to an optical coupler 63
And received by the upstream light receiver 42 of the head end 40 via the upstream trunk transmission line 52. Thus, the upstream signal of each area on the subscriber side is transmitted to the center side.

Also in this case, it is desirable that the wavelengths of the respective light sources be different from each other as much as possible in consideration of the optical beat noise. RIN value is -13
The influence of optical beat noise is avoided by setting the offset frequency so as to secure a value of 0 dB / Hz or less.

Accordingly, in the present embodiment, similarly to the second embodiment, deterioration of transmission quality is prevented, the total transmission distance of the transmission line and the number of head end components are reduced, and maintenance of each component is also easy. It becomes possible to.

[0033]

As described above, according to the present invention, light from a plurality of transmission units is modulated by a plurality of different frequency-multiplexed electric signals, output as optical signals, and combined with the optical signals. In an optical transmission method in which the light is transmitted to an optical transmission line, received by a light receiving unit, and demodulated, each of the transmitting units determines a wavelength interval of the light based on an effective modulation degree, for example, when the effective modulation degree is 4
In the case of 0% or less, and the difference between the frequency of the near electric signals to most optical beat the center frequency of the center frequency of the optical beat noise electrical signal set away above 20GH _Z, it outputs the optical signal Therefore, it is possible to define the wavelength interval of each light source with respect to a certain intensity modulation level, and to prevent deterioration of transmission quality.

When the effective modulation factor is 30% or less, for example, the wavelength interval of the light is the difference between the center frequency of the optical beat noise and the frequency of the electric signal closest to the optical beat center frequency among the electric signals. since the difference is set away above 15GH _Z, which also can prevent degradation of transmission quality.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment in which an optical transmission method according to the present invention is used in an optical ITV system.

FIG. 2 is a waveform diagram showing a relationship between optical beat noise and an electric signal when an effective modulation degree is 40%.

FIG. 3 is a waveform diagram showing a relationship between optical beat noise and an electric signal when the effective modulation degree is 30%.

FIG. 4 is a relationship diagram showing a relationship between a change in RIN and a frequency of optical beat noise.

FIG. 5 is a configuration diagram showing a second embodiment in which the optical transmission method according to the present invention is used in an optical CATV system.

FIG. 6 is a configuration diagram showing a third embodiment in which the optical transmission method according to the present invention is used in an optical CATV system.

FIG. 7 is an array diagram showing an example of a frequency array of signals of the third embodiment shown in FIG. 6;

[Explanation of symbols]

 1 to n optical transmission device 10 to 12 signal generator 13 to 15, 19 modulator 16 mixer 17, 41, 64 light source 18 optical coupler 20 center station 21, 42, 62 light receiver 22 distributor 23 demodulator 30, 51 , 52 Optical fiber 40 Head end 65 Diplex filter 66 Coaxial cable 71-7n Area

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Tomomichi Matsunaga Inventor 2-6-1 Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd. (72) Inventor Hideyuki Omura 2-6-1 Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd.

Claims (4)

[Claims] An optical signal from a plurality of transmission units is modulated by a plurality of different frequency-multiplexed electrical signals and output as an optical signal, and the optical signals are combined and transmitted to an optical transmission line. An optical transmission method for receiving and demodulating an optical signal, wherein each transmission unit sets the wavelength interval of the light based on an effective modulation factor and outputs the optical signal. 2. The wavelength interval of the light, when the effective modulation degree is 40% or less, is a difference between the center frequency of the optical beat noise and the frequency of the electric signal closest to the optical beat center frequency among the electric signals. 2. The optical transmission method according to claim 1, wherein the difference is set so as to be apart by 20 GHz or more. 3. The wavelength interval of the light, when the effective modulation degree is 30% or less, is the difference between the center frequency of the optical beat noise and the frequency of the electric signal closest to the optical beat center frequency among the electric signals. The optical transmission method according to claim 1, wherein the difference is set so as to be separated by 15 GHz or more. 4. The transmission section according to claim 1, wherein, of the electric signals of different frequencies for modulating light, a difference between a frequency closest to a center frequency of the optical beat noise and a center frequency of the optical beat noise is determined. 4. The optical transmission method according to claim 2, wherein a wavelength interval is set.